10 
PALAEONTOLOGY OF NEW YORK. 
posterior margin of the aperture is widely separated from the preceding volu- 
tion. The longitudinal ridges are more strongly marked, and of a different 
character from those of characteristic specimens of P. bucculentum; it is not, 
however, improbable that we may find intermediate forms uniting the two 
species. It is also possible that a comparison of a larger number of speci¬ 
mens may prove that the forms included under P. Thetis , P. symmetricum and P. 
bucculentum, are varieties of one species. 
Formations and localities. In the Upper Helderberg limestone at Darien, and 
in the Hamilton group at Darien, York, and Canandaigua lake, N. Y. 
Platyceras bucculentum. 
PLATE III, FIGS. 7, 26-29. 
Platyceras bucculentum, Hall. Descriptions of New Species of Fossils, etc., p. 5. 1S61. 
“ “ “ Fifteenth Rep. N. Y. State CaU. Nat. Hist., p. 33. 1862. 
“ “ “ Illustrations of Devonian Fossils: Gasteropoda, pi. 3. 1876. 
Shell ventricose, obliquely subovoid. Apex extremely attenuate, the spire 
making one or two closely enrolled volutions, with a gently enlarging 
diameter, and below this abruptly expanding and becoming very ventricose 
in the middle and lower part; spreading more upon the right side than 
upon the left; the shell near the posterior side swells out into a distinct 
poucli-like projection, with two or three rounded folds or semiplications, 
which give a deeply sinuous outline to the margin. Aperture subovate, 
and sinuate on the right posterior side. Peristome sinuous, and on the 
posterior side spreading partially over the preceding volution. 
Surface marked by fine closely arranged concentric striae, which are undulated 
towards the margin of the aperture, and sometimes over the greater part 
of the surface, the irregularity having commenced during the earlier stages 
of growth. In well-preserved specimens there are revolving striae or 
fascicles, rising in little bands of obsolescent striae, giving a waved aspect 
to the surface. 
This species is more ventricose than any of the preceding forms. The char¬ 
acter of apex, and the widely expanded body-volution resemble in some degree 
